1. Field of the Invention
The present invention relates to an image processing apparatus comprising judgment means for judging a dot photograph image, a photograph image or a character image and more particularly, to an image processing apparatus comprising judgment means for judging a dot photograph image, a photograph image or a character image based a counted value of a number of pixels each having a peak density in one-dimension direction larger than a predetermined threshold density (each pixel is referred to as a peak density pixel of one dimension hereinafter) and a counted value of a number of pixels each having peak densities in two-dimension directions larger than the predetermined threshold density (each pixel is referred to as a peak density pixel of two dimensions hereinafter).
2. Description of the Prior Art
The modulation transfer function (MTF) correction process provided for improving reproductivity of fine lines and preventing generation of Moire and the binarizing process for converting multi-value image data to binary image data in representation of pseudo gradation may lead to relatively large influence to the quality of image to be processed. Since binarizing processes suitable for attributes of documents of a character image, a photograph image and a dot photograph image are different from each other upon performing the above-mentioned processes, one suitable binarizing process is selected among these binarizing processes based on judgment results obtained by judgment of the attribute of the document. This selection has been used in an image processing apparatus such as a digital copying machine or the like. Therefore, the technique for detecting the attribute of the image located in a predetermined area within the document is very important.
Conventionally, a method of detecting a dot photograph image area using an autocorrelation function is suggested in the Japanese patent laid-open publication No. 59-182670/1984. In this detection method, first of all, there is calculated a coefficient Ps(i, j) of an autocorrelation function of image data located in a predetermined image area using the following equation. ##EQU1## where Is (x, y) is image data in an n.times.m matrix.
If the calculated coefficient Ps(i, j) is equal to or larger than a predetermined threshold value, the image of the image data is judged as a dot photograph image area. On the other hand, if the calculated coefficient Ps(i, j) is smaller than the predetermined threshold value, the image of the image data is judged as an area other than the dot photograph image area.
Further, there is suggested another method of detecting a dot photograph image area in the Japanese patent laid-open publication No. 63-28174/1988. In this method, there is detected a pitch of a dot photograph image using the above-mentioned method, a set of filtering coefficients for a spatial filtering process is selected among a plurality of sets of filtering coefficients based on the detected pitch, and then, a spatial filtering process is performed using the selected set of filtering coefficients.
However, in the above-mentioned methods each of detecting a dot photograph image area, it takes an extremely long time to perform the above-mentioned calculation, and it is necessary to provide a memory having a relatively large memory capacity.